fix(core): preserve SSH gateway default ports

[TaylorMutch]

Summary

Preserve the externally reachable default HTTP/HTTPS port when resolving the SSH gateway URL from a cluster endpoint. This prevents openshell sandbox connect from falling back to the server internal bind port when the gateway URL omits an explicit default port.

Related Issue

None.

Changes

• Use Url::port_or_known_default() when resolving the SSH gateway cluster port.
• Update resolver tests for default HTTPS ports and add coverage for a default HTTP endpoint.

Testing

• cargo test -p openshell-core resolve_ssh_gateway passes
• mise run pre-commit passes (fails in upstream rust:lint on unrelated crates/openshell-sandbox/src/process.rs:23 unused import: warn)
• Unit tests added/updated
• E2E tests added/updated (not applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)